1. Field of Invention
The present invention relates to a power supply, and more particularly, the present invention relates to a start-up circuit.
2. Description of Related Art
Switching mode power supplies have been widely used to provide regulated power for computers, home appliances, communication equipments, etc. In recent years, the problem of power saving in switching mode power supplies has drawn much attention. Based on the restriction of environmental pollution, the computer and other equipment manufactures have been striving to meet the power management and energy conservation requirements.
FIG. 1 shows a traditional approach for filtering electromagnetic interference (EMI) and providing a DC voltage. An EMI filter located between a power source VAC and a bridge rectifier 10 includes a choke L1, X-capacitors C1 and C2. The X-capacitor C1 is placed across the power source VAC. The choke L1 is coupled between the power source VAC and the bridge rectifier 10. The X-capacitor C2 is coupled between the choke L1 and an input of the bridge rectifier 10. A bulk capacitor CIN connected from an output of the bridge rectifier 10 to a ground is for stabilizing the DC voltage VBUS at the output of the bridge rectifier 10. For safety regulations in US and European, a bleeding resistor RD is generally placed across the X-capacitors CI or C2 of the EMI filter. The bleeding resistor RD will discharge the stored energy at the X-capacitors C1 and C2 to prevent an electric shock when end-user cut off the power source VAC. In fact, the bleeding resistor RD always has a fixed power-loss as long as the X-capacitors C1 and C2 have the stored voltage. Besides, for higher power source, the bleeding resistor RD consumes much standby-power when the power supply is operated at no-load. Therefore, the disadvantage of the traditional approach causes a poor power saving at light-load and no-load. Because of the existence of the X-capacitors, it has become a major concern to reduce the standby-power.